open channel flow meter 1

undrgrndmafia · Dec 8, 2015

Hello everyone
First of all i have to say, i have no knowledge on fluid mechanics. I am encountering a problem with designing a open channel flow meter (through reading level of the fluid from drain). I am intending to set it up on a open drain (12 inch wide and 24 inch deep). While reading theories i found out i need to form a rectangular weir or a notch. I am wondering the drain i also rectangular shaped and without any modification it could also serve the purpose or not? any help would be appreciated.

Thanks

cvg · Dec 8, 2015

maybe, maybe not, depends on where the hydraulic control section is located and if the water is flowing at normal depth or not. you will also need to determine if your flow is subcritical or supercritical.

suggest the following reference might be useful

http://www.usbr.gov/tsc/techreferences/mands/wmm/WMM_3rd_2001.pdf

bimr · Dec 8, 2015

No, you can not use a drain for a flow meter.

The standard size flowmeter channel shapes have been empirically correlated to the flow.

If you use a standard weir shape, the depth of a weir is also empirically correlated to flow. You would be better served using a prefabricated weir.

http://www.spitzerandboyes.com/wp-content/uploads/2014/02/FlowLikeAnEgyptian.pdf

Flow meters need to be set up so that backwaters do not interfere with the level measurements.

someguy79 · Dec 8, 2015

Here's a reference I've found useful. It's not as nice as an engineered solution from some of the flume suppliers; but could work to get started.
[URL unfurl="true"]http://www.engineeringtoolbox.com/weirs-flow-rate-d_592.html[/url]

undrgrndmafia · Dec 10, 2015

@cvg
Those terms are over my head

. Can you please rephrase that?
@bimr
Thank you for your answer. As you said 'standard size', i am unable to find any. A level vs flow graph can be calculated in any channel. For instance,
q = 3.33 (b - 0.2 h) h3/2 in francis formula of calculating discharge, i can put 'b' as width of the channel and 'h' height of water (measured by level transmitter) to have a discharge. You can consider it Sharp-crested, Suppressed type weir.
what i am not getting by not setting up weir.

cvg · Dec 10, 2015

Normal Depth

http://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Defining_NormalDepth.htm

good primer for hydraulic control, normal depth, super and sub-critical flow

http://mysite.du.edu/~etuttle/tech/opench.htm

this is a good in-depth publication

http://pubs.usgs.gov/of/1988/0707/report.pdf

bimr · Dec 10, 2015

undrgrndmafia,

There are a number of considerations. The flow entering the metering flume needs to have a transition into the flume. There has to be a transition on the discharge of the flume. The metering flume is designed such that the flow will be laminar with a level surface, not turbulent.

It is much easier to buy a prefabricated flume than to try to construct one.

http://www.plasti-fab.com/plasti-fab-products/

KevinNZ · Dec 13, 2015

You could use any shape channel you like and then site calibrate water level vs flow. You will need an other way to measure flow during the calibration.

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open channel flow meter 1

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